Is it possible to use a related field as a choice field in Django?-CodePudding

In Django, if I have something like this:

class Library(models.Model):
    name = models.CharField(...)
    address = models.CharField(...)
    book_of_the_week = ?

class Book(models.Model):
    library = models.ForeignKey(Library, on_delete=models.CASCADE, related_name="books")
    name = models.CharField(...)

This gives me the ability to create multiple libraries, each with a large number of books.

For book_of_the_week, I want this to be a reference to a Book instance, so that in Django Admin, the field is represented as a dropdown that lets you select from the books in the current library, and in code, you can use .book_of_the_week to access a specific instance of Book.

Is this possible?

CodePudding user response：

Sure, it's possible. But if you do this, you'll only ever be able to save the current book of the week. What happens if you want to show a library's book-of-the-week history? Seems like a likely scenario to me. Consider doing something like:

class Library(models.Model):
    name = models.CharField(...)
    address = models.CharField(...)
    book_of_the_week = ?

class Book(models.Model):
    library = models.ForeignKey(Library, on_delete=models.CASCADE, related_name="books")
    name = models.CharField(...)

class BookOfTheWeek(models.Model):
    book = models.ForeignKey(Book, on_delete=models.CASCADE, related_name='book_of_the_week')
    library = models.ForeignKey(Library, ...etc)
    week_of = models.DateField()

In this manner every book of the week will be a discrete database record, and you'll be able to track a history of each library's book of the week over time.

However...

That said, if you do not need to do this and a single instance record is fine, you should be able to create a ForeignKey relation from Library to Book. However, you may run into some circular and/or hierarchical reference issues depending on how where your model classes are written. In the example you show above, you'd need to declare your ForeignKey model name as a string, like so:

book_of_the_week = models.ForeignKey('Book', on_delete ... etc)

instead of:

book_of_the_week = models.ForeignKey(Book, on_delete ... etc)

...otherwise Django will throw an error because the Book model class is referenced before it is defined. Making 'Book' a string will let Django parse the full models.py file, build the logic, and avoid the error.

Sidenote:

Depending on your specific case you may also consider a ForeignKey to self, like so:

class Book(models.Model):
    library = models.ForeignKey(Library, on_delete=models.CASCADE, related_name="books")
    name = models.CharField(...)
    book_of_the_week = models.ForeignKey('self', on_delete=models.SET_NULL, ...etc)

This would allow any Book on the database table to reference another single Book instance.